1. Field of the Invention
The present invention relates to a termination for an electric cable, for medium and high voltages, of the type for outdoor use, exposed to air and possible polluting agents, complete in all its parts before being transported to installation place and suitable for connection to the electric cable without accessing into the inside thereof.
For the purposes of the present invention, with medium and high voltages it is generally meant voltages in a range of from 10 kV to 245 kV and more.
For the purposes of the present invention, the term "termination for an electric cable" means a device suitable to connect an electric cable to a bare conductor, such as, for example, an overhead conductor. This termination comprises electrical connecting means between the cable conductor and the bare conductor, and electrical separation means between the outer surface of the cable and parts connected thereto, generally at ground potential, and the elements under tension, such as the bare conductor. A termination of this kind generally comprises a conductive element, an insulating coating, and field control means able to produce an electric field configuration compatible with dielectric strength features of the relevant elements.
2. Description of the Related Art
A known termination for outdoor use is described in the publication "New Prefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cables" (Underground Transmission and Distribution Conference, 1974, pages 224-232).
The termination comprises a supporting base for an outer insulating casing made of porcelain and provided with a finned surface. The electric cable devoid of its shield extends over the whole length of the termination up to the upper extremity where it is fastened to be supported by the casing and where connection occurs to a voltage shank and therefrom to another electric installation.
A cylinder made of an epoxy resin incorporating a ground electrode is assembled about the elastomer element and is fastened to the supporting base. The cylinder has a conical shape matching that of the cone-shaped element inside it.
The installation further comprises the use of a spring system acting on the lower end of the elastomer element so that a pressure between such element and the cable insulation takes place and also between the element and the epoxy resin cylinder, so as to avoid the presence of air traces and thus the risk of electrical discharges.
The space inside the porcelain casing is filled with insulating oil.
The termination, due to the presence of a porcelain casing, is of the self-bearing type and rests on a base being part of a pylon or the like.
Also known are terminations provided with a solid insulator of the overhead type instead of resting on a supporting structure. According to this solution the termination is associated with the structure by interposition of an insulating element.
For example, from publication "IEE Power Cables and Accessories 10 kV-180 kV", London, November 1986, pages 238-241, it is known a 66 kV voltage termination for an electric cable which is extended over the whole length of the termination.
In this solution, starting from one cable end the semiconductive shield is removed over a certain length thereof and the shield end is coated with a conductive paint until an insulator portion is covered.
On the area where the shield is cut off, heat-shrinkable tubes of given impedance due to the presence of capacitive and resistive elements are applied, which tubes capacitively associated with the cable conductor reduce electrical stresses in the area where the shield is cut off.
An additional shrinkable tube is then disposed about the innermost tubes of same type and about the cable insulator portion.
The outermost tube comprises an insulating profile close to the cutting-off end of the cable shield.
A plurality of fins of heat-shrinkable insulating material is then fitted on the outer tube.
For installation of the termination qualified workers skilled in heat-shrinking technique are required and accuracy, attention to details and cleanliness in all steps are needed.
Patent WO 91/16564, in its part describing the state of the art, mentions terminations made, as the previous one, according to heat-shrinkage technique, and explains how to avoid that technique by making use of a suitable mandrel on which a jacket of elastomer material provided with finnings is expanded; the jacket is contracted by slipping off the mandrel onto a previously prepared electric cable of smaller diameter.
The final cable portion to be enclosed within the termination comprises the cable conductor, followed at a certain distance by the insulator devoid of a semiconductive layer and the shield.
A tubular winding is part of the cable assembling, made of a material formulated so as to reduce stresses otherwise present at the end of the semiconductive layer; this winding is applied on the semiconductive layer and the insulator.
Alternatively, provision may be made for a layer intended for reducing said stresses within the inner insulating jacket, by coextrusion or rolling for example.
The upper end of the conductor is associated with a flat clamp provided with a hole.
To make slipping the mandrel off easier, a plurality of ribs may be provided on which the elastomer jacket rests, and also the use of a lubricant between the ribs.
The mandrel is disposed with its elastomer jacket about the cable end portion already arranged to be part of the termination. The jacket twisting on the mandrel facilitates the lubricant spreading on the ribs and the mandrel slipping off, thereby causing contraction of the jacket on the cable end portion.
Also known are terminations for electric cables in which the electric cable is inserted only partially into the termination instead of extending up to the shank connecting to the overhead line or other electric installation.
One example of such self-bearing termination is described in patent application EP 95 101 338.2 of the Applicant itself.
In this solution, the electric cable is stopped at a given point from the termination entrance and the electric connection over the whole length which is required to reach the overhead line is accomplished by means of a conductive element.
The conductive element is externally provided with a finned elastomer layer and is supported by an insulating base preferably made of an epoxy resin resting on a supporting element.
The assembly consisting of the epoxy resin base and the conductive element makes the termination of the self-bearing type.
In particular, the assembly consisting of the epoxy resin base and the rigid conductive element, brings the electric field to the termination surface and to a value compatible with the dielectric strength of air that is long enough to form a leak line adapted to resist discharges.
The electric cable, at a given distance from the termination entry, is deprived of the shield, leaving the insulator uncovered. At the shield cutoff, electric field control means are provided which consist of a deflecting cone and an upper insulator useful for giving rise to electric field values compatible with the electrical strength of the employed materials and the surrounding air.
The epoxy resin base comprises a cone-shaped cavity on which the end portion of the insulator of the field control means rests.
A suitable spring arrangement pushes the cone and the insulator over it against the cone-shaped surface of the resin base in such a manner that no incorporation of air bubbles may occur between the contact surfaces of the different parts, which as known may bring about risks of electrical discharges.
WO 97/09762 discloses a cable termination for a high-voltage cable insulated by a solid insulating material which is used to connect the cable with an insulated conductor. With a particular arrangement of electrical couplings and field-controlling elements between the cable conductor and the insulated conductor, this device allows a rapid and leakage-free installation forming a closed current path without the risk of flashover. That device is primarily (even though not exclusively) designed for connecting solid-insulated cables with gas-insulated devices. In particular, in FIG. 2 a connection between a solid-insulated cable and a gas-insulated insulator for outdoor use is illustrated.